Real-Time Probabilistic Prediction of Storm Water Level At Japanese Ports

Every year, several typhoons approach the Japanese Islands and a few make landfall. Among recent typhoons, Typhoon Bart in 1999 brought a storm surge of approximately 3.5 m into Yatsushiro Bay, Kyushu Island, near the time of a spring high tide. The storm surge triggered flooding water levels, reaching the roofs of one-story houses on low-lying land, and took 12 lives in these houses. Fig. 1 shows the variation in the storm water level with time at the affected place. It was difficult to identify the difference between the actual storm water level and the usual astronomical tide level until 5:00 a.m., as well as to imagine that the storm water level had risen by 4 m until 6:30 a.m. owing to a significant storm surge and the usual rise in astronomical tide level. From such events, we should recognize the necessity of storm water level prediction for disaster prevention work, such as tide gate operation, sand bag piling and evacuation orders, before the storm water level reaches the point of disaster. Meteorological organizations not only in Japan but also in the other countries in the world issue the most probable typhoon track with the range of the standard prediction error. The persons in charge of disaster prevention work carry out the work if they feel there is a certain high risk of flooding; however, they do not know the probability that a higher-than allowable-water level will manifest. A numerical meteorological model, however, has a certain range of uncertainty in typhoon track prediction. Meteorological organizations, thus, provide the most probable typhoon location at several lead times, such as 12, 24, and 48 h, as well as indicate the standard estimation error. The Japan Meteorological Agency (JMA) uses the prediction error circle, which the typhoon center enters with a probability of 70%, and assumes 5 typhoon tracks, which trace the center, leftward, rightward, forward and backward points, respectively, of the prediction error circle at each lead time. Then, the agency conducts a storm surge simulation for these typhoon tracks, estimates the storm water level as the sum of the storm surge and the astronomical tide level, and has the responsibility of issuing a storm surge warning to the public if the storm